This invention relates to a paper feed mechanism for a printing device of the electrophotographic type.
In a conventional paper feed mechanism for a printing device of the electrophotographic type, a number of paper sheets stacked on a paper stack tray 2 are fed individually by a pickup roller 3 as shown in FIG. 6. An uppermost one of the stack of paper sheets is fed first. The primary advantage of this paper feed mechanism are the accuracy of the paper feed, and the prevention of a double paper feed. One known method of positively feeding a number of stacked paper sheets one by one from the upper side employs a torque limiter. In this case, the paper sheet 1 is fed by the pickup roller 3, and the double paper feed is prevented by a feed roller 4 and separation roller 5 disposed downstream of the pickup roller. The separation roller 5 has the same axis of rotation as a torque limiter 7 mounted on a non-rotatable shaft 6. The separation roller 5 is connected to the torque limiter 7 via a coupling, and is pressed against the feed roller 4 by a spring 10.
Because of the provision of the torque limiter 7, the separation roller 5 will not be rotated if it does not receive a certain amount of load. Namely, as the load on the separation roller 5 increases, the separation roller 5 rotates together with the feed roller 4, and the two rollers rotate in the same direction at the point of contact therebetween.
Generally, the coefficient of friction between paper sheets is smaller than the coefficient of friction between a paper sheet and a rubber roller. Therefore, when two paper sheets are interposed between the separation roller 5 and the feed roller 4, the load on the separation roller 5 is relatively small, so that the separation roller 5 is not rotated to stop the paper sheet in contact with the separation roller 5. On the other hand, either when no paper sheet exists between the separation roller 5 and the feed roller 4, or when one paper sheet exists between the two rollers, the load exerted on the separation roller 5 is relatively large, so that the separation roller rotates together with the feed roller 4 to feed the paper sheet in cooperation with the feed roller 4 if the paper sheet exists between the two rollers.
In this manner, the paper separation is carried out by the separation roller 5 and the feed roller 4. In order that this operation can be carried out properly, a certain relation must be established between the torque of the torque limiter 7 and the pressing force of the separation roller 5.
This relation is expressed by the following formula: EQU T/.mu.R&lt;N&lt;T/.mu.P-(2M+3m) (1)
T=.tau./r PA1 .tau.:torque of the torque limiter PA1 r:radius of the separation roller 5 PA1 N:pressing of the separation roller 5 PA1 M:pressing force of the pickup roller against the sheet PA1 m:weight per paper sheet PA1 .mu.R:friction coefficient between the sheet and the roller PA1 .mu.P:friction coefficient between the paper sheets.
FIG. 7 is a graph representing the formula (1). In order for the paper sheet to be positively separated, it is necessary that the relation between T and N should be in the region enclosed by a line of N=T/.mu.P-(2M+3m) and a line of N=T/.mu.R. Generally, T has a constant value, and therefore the value of N is adjusted so as to satisfy the formula (1).
In order to enhance the accuracy of paper feed and the reliability of prevention of double paper feed, the environment, the aging change and variations in parts must be taken into consideration. Generally, .mu.P increases when the temperature and moisture become high, and .mu.R decreases with age. Therefore, it is necessary that the adjustment of N should be made considering the maximum value of .mu.P and the minimum value of .mu.R, and this range is narrow as clearly seen from FIG. 7.
In view of variations in the torque limiter 7, it is possible that the adjustment range of N is further narrowed. For example, in FIG. 7, assuming that T is T0 and that its tolerance is .+-..DELTA.T, and the adjustment range is N1&lt;N.ltoreq.N1' in the case of T=T0-.DELTA.T, and the adjustment range is N2&lt;N.ltoreq.N2' in the case of T=T0+.DELTA.T. Then, in the case of N2.ltoreq.N1, there exists N which satisfies the formula (1) in the range of T=T0+.DELTA.T. However, a value of N which satisfies all of these equations exists only in the range of N2&lt;N.ltoreq.N1', thus the operation region is decreased. In contrast, in the case of N2&gt;N1', there does not exist a value of N which satisfies the formula (1). If N is adjusted to the range of N1&lt;N.ltoreq.N1', the paper sheet can not be fed by the separation roller 5 and the feed roller 4 in the case of T=T0+.DELTA.T. Also, if N is adjusted to the range of N2&lt;N .ltoreq.N2', the double paper feed may not be prevented in the case of T=T0-.DELTA.T.
Thus, there it is problematic that the adjustment range of N is easily influenced by the values of .mu.P, .mu.R and T and therefore a satisfactory value of N can not easily be determined.